The invention is related generally to thin films of low density materials and more particularly to thin films of amorphous boron, which can be used as a hard surface material or can be layered with other materials for use in optical coatings or as a hardening material, for example.
Thin foils and thin films are widely used as band-pass filters, in transmission filters, and in spectroscopic applications which use irradiation wavelengths in the range of extreme ultra-violet to soft x-rays. Thin foils and thin films of boron are also desirable for use to increase the hardness of a surface or of a material. Submicron foils, 0.1-0.2 xcexcm thick, have been produced in various low Z materials (low atomic number) (i.e. carbon, beryllium) by sputtering and evaporation processes. Boron films, however, have been produced by resistance heating or electron beam evaporation (Labov, S. et al., Applied Optics 24: 576 (1985)). Previous efforts to prepare boron foils by sputter deposition were precluded by the non-availability of dense, high purity sputter targets. Typically sputtered films exhibit superior mechanical properties and are preferred because they have fewer defects and finer morphological growth features than foils or films prepared by evaporative processes.
Multilayered coatings, which are used as reflective layers in x-ray optics, are typically tens to hundreds of angstroms thick. A multilayer x-ray mirror is the analog of a quarter-wave stack reflective coating with the added complication of radiation absorption in the layers. Physically, it is an alternating sequence of thin films of highly absorbing and less absorbing materials deposited on an optically smooth substrate. The layered structure is periodic and results in a large angle, resonant reflectivity which is three or four orders of magnitude greater than the simple Fresnel reflection from an unlayered surface. Reflectivity in a multilayer mirror derives from the interference of x-rays coherently scattered from the interfaces between materials of higher or lower x-ray absorption.
The quality of the multilayered optical coating is determined by the perfection of the interfaces between the layers and the uniformity of the layer dimensions. Standard methods for application of multilayer coatings use the physical vapor deposition (PVD) process of evaporation or sputtering.
The coarse layer microstructure produced and the inherent difficulty in controlling the evaporation processes adversely effects the interface perfection and layer dimensional stability, and consequently, the efficiency of the optical coating produced by such methods. The use of computer controlled sputtering processes allows the production of complex multilayer coatings with variable layer thickness and composition.
Accordingly, it is an object of the invention to provide thin, amorphous boron films which have no morphological growth features.
It is a further object of the invention to provide thin multilayer structures which utilize thin boron films.
It is another object of the invention to provide multilayer structures which comprise thin boron films with high Z metal interlayers.
It is another object of the invention to provide a material of increased hardness using thin boron films.
The invention provides thin boron foils which do not include any morphological growth features such as columnar boundaries. These ultra-thin films can be produced as free-standing filters which do not require a protective or supportive layer, such as carbon-hydrogen based polymer films, which would reduce or contaminate the transmitted radiant energy intensity. Also, the ultra-thin boron films can be deposited on materials such as used in various types of tools to increase the hardness and thus increase the wear resistance of such tool materials.
A foil is a very thin sheet of metal, which is usually not thicker than 0.15 mm. A thin film is a material which may be on a substrate, with a thickness not greater than 10 xcexcm and uniformity within 20% of its average value. In the instant application, the terms thin film of metal, or thin film and the term foil will be used interchangeably to represent one layer of a particular metal which may be selected from a range of metal thicknesses. In the instant application, low Z refers to metals with an atomic number of 20 or less, and high Z refers to any metal or a group of metals with an atomic number greater than 20, which includes transition metals, refractory metals, or noble metals.
The stability of the amorphous sputter-deposited boron suffices for the formation of layered structures even in reactive and energetic binary systems. With the use of these thin boron layers as the non-absorbing low Z (low atomic number) layer in high Z/low Z multilayer structures, it is possible to further reduce the absorption of such structures below that which is attainable with other low Z layers, such as carbon or boron carbide (B4C) layers. This thin boron layer fabrication technique is applicable to the production of multilayer components, such as mirrors, for use with a wide range of optical wavelengths, and well as the production of hardened materials.
The fabrication of boron foils, films and multilayered structures has, previously, not been possible because of the problem of securing sufficiently dense, high purity boron to serve as a target for sputter deposition. A suitable target is now available as a high density, crystalline boron prepared by a method described by Hoenig et al. in Proceedings of the Seventh CIMTEC World Ceramics Congress, Montecatini-Terme, Italy, Jun. 24-28, 1990, published by Elsevier, The Netherlands, which is incorporated by reference. With this high density target material, it is possible to adequately control the sputtering process so that uniform thin amorphous boron foils, films and multilayer structures, can be produced.